This invention relates to terminations for electrical power cables, for example, operating at voltages in the range from about 35 kilovolts to 500 kilovolts or above.
When an electrical power cable is connected to an overhead line, a transformer, switchgear, or other equipment, a suitable termination is required to manage the transition of the load-carrying conductor from the environment of the cable in which it is surrounded by high-quality, solid, insulating material of high electrical strength with electrical stress geometry strictly controlled by a concentric earth screen to one in which insulation is mainly provided by air at relatively low stress concentrations and uncontrolled geometry, while protecting the cable end from contamination, and, where applicable, from leakage of insulating fluids.
Typically, these terminations comprise tubular member, usually of ceramic insulating material, to provide adequate spacing from the high voltage at the end of to earth potential at the screen of the cable. The cable conductor, or an extension of it, runs down the central longitudinal axis of the tubular member, with a stress cone or other means provided in the annular space between the conductor and the tubular member for controlling electrical stress concentrations. Resilient mechanisms may also be provided to ensure that interfaces are under pressure.
While these components fill most of the space inside the tubular member, there are still residual spaces which need to be filled with a material having better electrical propertiesxe2x80x94primarily breakdown strength, that is the electrical stress concentration at which ionisation occurs leading ultimately to electrical breakdownxe2x80x94than air, to reduce the risk of electrical breakdown at exposed surfaces. Typically, a suitable insulating fluid such as silicone oil or a compressed gas (for example sulfur hexafluoride) are used, but this increases the cost of making the terminations. It has also been known to use large, moulded, polymer shapes to occupy most of the space.
According to the present invention, there is provided a termination for an electrical power cable including a load carrying conductor, the termination comprising an insulating outer member through which the load carrying conductor, or an extension of it, runs substantially longitudinally, and means for controlling electrical stress concentrations within the outer member, characterised in that residual space within the outer member is occupied by electrically insulating solid granules and interstices between the granules are filled with electrically insulating fluid.